1. Field of the Invention
This invention relates to a video signal synchronization system for a television device and more particularly to a synchronization signal generator which permits timing and clock signals for a device to remain phase locked to an input video signal while providing a composite output video signal that is in proper phase relationship to a studio locking reference signal.
2. Discussion of the Prior Art
Commercial television studios typically have a switcher which is connected to receive up to 24 video input signals. These signals may be generated by a variety of sources including local cameras, video recorders, and remote cameras. A given video signal may be coupled to a first switcher input and simultaneously coupled through a special effects device to a second switcher input. The switcher may select, under operator control, any one of these input signals for output as the television broadcast signal.
The signal information for each horizontal line of a television video image contains three components which are particularly critical to proper operation of a television receiver. Each video line begins with a short horizontal sync pulse which operates to stabilize the phase and frequency of a horizontal oscillator in a television receiver. The horizontal sync pulse is followed closely by a color subcarrier or color burst signal. This signal component provides a color reference to help stabilize the color displayed by a receiver. This is a sine wave signal having a rather critical phase tolerance and a less critical tolerance as to the time window during which the signal component is generated. Finally the video data signal component itself is transmitted. Phase or time tolerances of plus or minus 70 nanoseconds for the start of the color burst component are tolerable. All of the above signal components and tolerances are defined by established standards.
If these three signal components are not within designated tolerances with respect to phase when the switcher changes from one input to another, television receivers will manifest undesirable characteristics such as rolling of the video image or color discontinuity. A significant effort is required to maintain all of the incoming signals to the switcher in tolerance. The studio generates a locking reference signal defining a reference horizontal sync pulse and color burst phase condition. This locking reference signal is presented to studio devices such as cameras and video recorders to permit these devices to operate in a common phase relationship. These devices further include manual phase adjustments for the horizontal sync pulse phase and the color subcarrier phase to account for varying factors such as different cable lengths so as to maintain the proper phase relationships at the switcher inputs. These manual adjustments must be frequently readjusted in order to maintain the many video signal components properly in phase at the inputs to the switcher.
A special problem is presented by a video signal manipulating device such as a video special effects system which must receive a signal from another source, transform or operate upon the signal in some way and then present the transformed signal to the switcher.
Typically a signal generator such as a camera is synchronized by the master sync signal and synchronously provides a generated video signal as one input to the video switcher. The video signal is then coupled in parallel through an auxiliary switcher to a transform device such as a digital special effects system. The output of the transform device is then coupled back as a second input to the switcher. The transform device may provide a nominal delay of one frame or some other integral number of frames so that the output is in phase with the input. However, the auxiliary switcher and cable delays as the signal passes through connecting cables impose phase delays which preclude the transformed video signal and the original video signal from both being properly phased at the video switcher. Furthermore, phase variations at other inputs to the switcher further increase the phase variations among all of the video signal inputs.
In prior systems this problem was solved by providing separate input and output clocking systems at the transforming video device. The input portion of the device can then be phase synchronized to the input video signal while the output portion is synchronized to the master sync signal. However, the provision of separate clocking signals is expensive and for complicated systems wherein the two clocking signals must be distributed over large printed circuit board surface areas, the phase differences of the clock signals frequently provide cross talk noise problems which result in unsatisfactory operation of the device.
The present invention solves this problem by operating the video device totally under control of a single pixel rate clock signal in synchronization with the input video signal. At the output, a sync generator responds to a phase difference between the input video signal and the master sync signal to properly adjust the phase of the output video signal relative to the phase of the input video signal to provide a properly phased output video signal. This adjustment occurs under control of the system input signal phase synchronized clock signal to avoid interference between clock signals of different phases.